I like “a subject of study or interest” better than “science”, because “logies” in a sense (1) also include astrology, phrenology and pyramidology. (One former colleague of mine used to say that there are real sciences, like chemistry and physics, and phony ones, which all end on “ology”.) There is a tendency (at least as far as renaming of university departments goes) to substitute well-established terms of Greek etymology with pluralised English equivalents: “earth sciences” instead of “geology”, “life sciences” instead of “biology” and so on. I can’t help thinking that, by using plurals, they try to get more funding. But what on earth is “biological sciences” if not a tautology? Does not biology sound scientific enough? Apparently not.

Since there are too many fields of study and people know only so many Greek words, it is inevitable that quite a few “logy” (1) terms are hybrid words which mix Latin and Greek parts. For example, sociology (the corresponding Greek word is κοινωνιολογία) and scientology (both contemporary and early usages of which sound tautological). Similarly, Italian, Russian and other Slavic languages have a hybrid word algologia (альгология) — phycology, a branch of botany dealing with algae. The (non-hybrid) English term algology means something else: study of pain.

Then there is philology. It does not mean “study of love” but the other way round: “love of study”. Really, it should be logophilia, and indeed such a word exists, meaning “love of words”, with a vague hint of a medical condition.

Apology and eulogy belong to the third group of “logies”. These two can be made into verbs (apologise, eulogise) — something you really cannot do with “logies” of (1) and (2) types. “Logy” (3) is very similar to -logue as in dialogue, monologue, catalogue, Decalogue...

What about analogy and homology? They don’t belong to any of these three categories. They are derived from the same Greek root logos but use another meaning of it: ratio, proportion.

Many European languages retained the Greek spelling, -logia of their -logies (see the table below). In English, logia (Greek λόγια), a plural of logion, refers to collection of sayings of Jesus, especially those which did not end up in the Gospels. In Spanish, logia (from Italian word loggia) means either lodge (as in Logia Masónica) or, er, loggia.